The xylene isomers are produced in large volumes from petroleum as feedstocks for a variety of important industrial chemicals. The most important of the xylene isomers is paraxylene, the principal feedstock for polyester, which continues to enjoy a high growth rate from large base demand. Ortho-xylene is used to produce phthalic anhydride, which supplies high-volume but relatively mature markets. Meta-xylene is used in lesser but growing volumes for such products as plasticizers, azo dyes and wood preservers. Ethylbenzene generally is present in xylene mixtures and is occasionally recovered for styrene production, but is usually considered a less-desirable component of C8 aromatics.
Among the aromatic hydrocarbons, the overall importance of xylenes rivals that of benzene as a feedstock for industrial chemicals. Xylenes and benzene are produced from petroleum by reforming naphtha but not in sufficient volume to meet demand, thus conversion of other hydrocarbons is necessary to increase the yield of xylenes and benzene. Often toluene is de-alkylated to produce benzene or selectively disproportionated to yield benzene and C8 aromatics from which the individual xylene isomers are recovered.
An aromatics complex flow scheme has been disclosed by Meyers in the HANDBOOK OF PETROLEUM REFINING PROCESSES, 2d. Edition in 1997 by McGraw-Hill, and is incorporated herein by reference.
Traditional aromatics complexes send toluene to a transalkylation zone to generate desirable xylene isomers via transalkylation of the toluene with A9+ components. A9+ components are present in both the reformate bottoms and the transalkylation effluent.
Paraxylene is most often produced from a feedstock which has a methyl to phenyl ratio of less than 2. As a result, the paraxylene production is limited by the available methyl groups in the feed. In addition, paraxylene production also typically produces benzene as a byproduct. Since paraxylene is more valuable than benzene and the other byproducts produced in an aromatics complex, there is a desire to maximize the paraxylene production from a given amount of feed. There are also cases where a paraxylene producer would prefer to avoid the production of benzene as a byproduct or paraxylene production. However, there are also cases where a paraxylene producer would prefer to limit the production of benzene as a byproduct or paraxylene production by making adjustments.